Reverse burning phenomenon in self-propagating high-temperature synthesis
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Reverse burning phenomenon in self-propagating high-temperature synthesis Jou-Hong Lee, Ai-Yi Lee, and Chien-Chong Chena) Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan (Received 27 January 1997; accepted 2 October 1997)
An interesting reverse burning phenomenon was observed during the combustion synthesis of zirconium-based materials. When an external heat was applied to one end of a green pellet, the ignition was initiated at the other end. Also, the ignition position, measured from the heated end, was proportional to the apparent green density of the compact. The possible explanations for this reverse burning phenomenon are discussed.
I. INTRODUCTION
The self-propagating high-temperature synthesis (SHS), or the combustion synthesis, is emerging as an attractive method to produce powders or densified products of advanced materials, because it has proven advantages1 : lower energy requirement, higher product purity, simpler and cheaper equipment, higher sinterability, and possible nonequilibrium phases in the products. In SHS, a reactant pellet is subjected to an external heat source, which can be a heated coil, laser beam, etc., on one end and is then ignited at this end. Once the pellet is ignited, the external heat source is removed immediately. Due to the highly exothermic reaction, a self-sustained combustion wave, which is also referred as a solid flame, propagates from this ignited end to the other and converts the reactant pellet into the final product. Many advanced materials1–5 such as refractories, intermetallic materials, super alloys, cermets, and superconductors can be synthesized by this method. Although the SHS method seems to be very simple, the underlying physicochemical mechanisms are actually complicated. The mechanisms involved include chemical reactions (solid, liquid, and gas phases could participate), melting, permeation, phase transitions, mass transfer, and heat transfer. Depending on the combustion conditions and the participating reactants, the combustion phenomena can be classified into the following categories: (i) steady combustion, the most common one: the velocity of the combustion wave front is constant throughout the reactant pellet; (ii) oscillatory combustion: the combustion front is flat and perpendicular to the wave motion, but moves in a succession of rapid and slow displacements6 : (iii) spin combustion: the combustion wave travels in a spiral motion from one end of the reactant pellet to the other end7 ; and (iv) aperiodic or chaotic combustion: time tracing of the wave front locations is aperiodic or chaotic.8
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http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 6, Jun 1998
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However, irrespective of which kind of combustion mode is involved in the combustion synthesis, the ignition always starts at the heated end; i.e., the ignition hot spot always appears first at the place which receives
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